This invention relates to methods and apparatus for monitoring the integrity of the environment associated with stored biological materials. More particularly, the present invention relates to a novel and improved method and apparatus for detecting that the supply of cryogenic fluid used in super-cooled storage of biological materials has not dropped below a predetermined amount. The invention is particularly useful for activating a detectable alarm whenever the supply of cryogenic fluid which generates a supercooled environment for biological materials has been reduced to a level which requires attention.
It has been known for some time that various biological materials can be stored in a supercooled environment and subsequently revived for future use without significant damage. Liquid nitrogen is a particularly popular refrigerant used in storage containers for such biological materials to maintain the temperature at about -320.degree. F. Liquid nitrogen is especially attractive for such applications because of its relatively inert gaseous qualities but, as with any cryogenic fluid, it will evaporate from a container at varying rates depending upon the type of container used. These containers are presently used extensively in the farm and ranch community for preservation of animal semen for use in artificial insemination. In addition, hospitals and medical research centers are employing cryogenic storage containers for many biological items such as tissue cultures, eye corneas, blood, human semen and the like. The monetary value of such biological materials is high and in many cases the materials are difficult to replace or even irreplaceable.
A variety of biological containers using high vacuum design have been developed. Upon loss of the vacuum or thermal insulation qualities, the length of retention time for the cryogenic fluids changes drastically such as from days or weeks to a matter of hours or even minutes. Complete evaporation loss of the liquid nitrogen from the container so that the biological materials are warmed even to an approach of ambient temperature can render the materials useless. This loss is further aggravated where containers must be transported via vehicles of various sorts and thus exposed to potential handling damage. Still further, the degradation of the thermal insulation qualities of the container is frequently difficult to detect and not noticeable until the refrigerant loss has progressed beyond the point of recovery.
The use of devices for detecting liquid level in accordance with the prior art such as dip-stick probes, static head weight pressure gauges, and the like require special and frequent attention as well as hazarding additional potential refrigerant loss. It has been known to monitor the contents of a container by inspection of the weight changes. For instance, U.S. Pat. Nos. 1,552,939 by Mackey and 3,656,138 by Hamma show spring biased scale arrangements with appropriately positioned electrical contacts associated with the spring scale pointers for closing an electrical alarm circuit. Unfortunately, spring scale devices are limited in their range of application and unreliable for many uses. Thus there has been a continuing need for a refrigerant monitoring system which can accommodate a wide variety of different containers and weight ranges in a reliable and accurate manner while providing an alarm whenever a preselected threshold level has been passed.